1. Technical Field
The present invention relates to a solid electrolytic capacitor using a conductive polymer as a solid electrolyte and a fabrication method thereof.
2. Description of Related Art
With recent reduction in size and weight of electronic devices, there is an increasing demand for a high-frequency capacitor which is small in size and high in capacity. For such a capacitor, a solid electrolytic capacitor has been proposed which has a dielectric layer formed by oxidizing a surface of an anode comprised of a sintered body of a valve metal, such as tantalum, niobium, titanium or aluminum, and a solid electrolyte layer disposed on the dielectric layer. In the effort to reduce an equivalent series resistance (ESR), a conductive polymer is used for the solid electrolyte layer.
However, deterioration of adhesion between the inorganic dielectric layer and the organic conductive polymer layer, if occurs, problematically increases leakage current and ESR.
Japanese Patent Laid-open Nos. Hei 2-74021, Hei 4-73924 and Hei 8-293436 propose to subject a dielectric layer formed on a surface of an anode to a surface treatment with a silane coupling agent before a conductive polymer layer is deposited.
Japanese Patent Laid-open No. Hei 11-329900 proposes to repeat a sequence of treating with a silane coupling agent and forming a conductive polymer layer onto the dielectric layer.
Japanese Patent Laid-open No. 2006-140443 proposes forming a first conductive polymer layer on a surface portion of a dielectric layer, treating the remaining surface portion of the dielectric layer with a silane coupling agent to form a treated layer and then forming a second conductive polymer layer over the first conductive polymer layer and the treated layer.
However, the amount of the silane coupling agent that reacts with or adsorbs onto a substrate varies depending on the surface condition of the substrate, specifically, depending on its hydroxyl content. Also because the silane coupling agent starts a reaction with a hydrolysis reaction by water, it is readily influenced such as by ambient moisture, which has been a problem.
Accordingly, there has been a need for a solid electrolytic capacitor which can be fabricated without using a silane coupling agent in the preceding fashions, has a high capacitance and exhibits low ESR and leakage current.